1. Field of the Invention
The present invention relates to piezoelectric thin-film filters, and more particularly, to a piezoelectric thin-film filter in which a piezoelectric thin film is sandwiched between electrodes and in which a BAW (bulk acoustic wave) resonator utilizing resonant vibration of the piezoelectric thin-film is used.
2. Description of the Related Art
In recent years, dual-mode piezoelectric thin-film filters to be used as RF filters of cellular phones have been developed. For example, Japanese Patent No. 3535101 discloses a device in which BAW resonators are disposed in a direction with a plane. As shown in FIG. 1, a supportive structure unit 2 provided on a substrate 1 supports a diaphragm 3 with a hollow space 4 therebetween, and a piezoelectric thin film 6 is disposed on the diaphragm 3. On both main surfaces of the piezoelectric thin film 6, a plurality of electrode fingers 5a and 7a of a first electrode pair and a plurality of electrode fingers 5b and 7b of a second electrode pair are disposed alternately with gaps therebetween so as to face each other across the piezoelectric thin film 6. The electrode fingers 5a and 7a of the first electrode pair are connected to input terminals, and the electrode fingers 5b and 7b of the second electrode pair are connected to output terminals. Vibration generated by a resonator formed between electrode fingers 5a and 7a of the first electrode pair is propagated to a resonator formed between adjacent electrode fingers 5b and 7b of the second electrode pair, and an electric signal is output. In this device, the impedance ratio of an input terminal to an output terminal is 1:1.
In order to achieve mechanical coupling between resonators in such a piezoelectric thin-film filter, a pitch (center-to-center distance P) between electrode fingers 5a and 7a of the first electrode pair and adjacent electrode fingers of the second electrode pair must be reduced. For example, the fourteenth paragraph of Japanese Patent No. 3535101 discloses that, in order to reduce the pitch (center-to-center distance P) between the electrode fingers 5a and 7a of the first electrode pair and the electrode fingers 5b and 7b of the second electrode pair, it is advantageous to set the width W of each of the electrode fingers 5a, 7a, 5b, and 7b to be smaller than a value twice the thickness T of the piezoelectric thin film 6, that is, to set the value such that W<2T is satisfied.
However, when the width of each of the electrode fingers 5a, 5b, 7a, and 7b is reduced, the bandwidth of the piezoelectric thin-film filter is reduced.